What does the Battle of Puebla represent to Mexican Americans? (PP Link)
Why do SER and IR have the same form in the preterite?
First, it is really the perfect (= Spanish preterite) conjugation of the Latin verb ESSE “to be” which came to be used for both ser and ir in Spanish. In Spoken Latin, the preposition IN (> Spanish en) used with a location came to mean “movement toward”. Also, ser and estar did not have the same restrictions in Old Spanish that they do in modern Spanish and consequently ser occurred at times where one might expect estar. Thus FUERUNT IN CAMPUM, for example, originally meant “they were at the countryside”, but later “they were moving/in route toward the countryside”, and eventually came to mean “they went to the countryside”. Therefore, this paradigm, fui, fuiste, etc., eventually became associated with the infinitive ir “to go”. http://www.virginia.edu/cla/avd/linguistics-for-spanish-teachers/history/
Or, to simplify, verbal constructions meaning "being at" a place and "going to" a place became confused. Thus, someone might have said something like, "Yesterday, I was in Salamanca." Someone else might express pretty much the same idea by saying, "Yesterday, I went to Salamanca." Nowadays, the language has settled on *Estar* to indicate location, but Old Spanish could use *Ser*. The real mysteries about this are as follows:
Pretérito de (Preterite tense of) SER (to be) IR (to go)fui (I was) fui (I went)fuiste (you were) fuiste (you went)fue (he/she/it was) fue (he/she/it went)fuimos (we were) fuimos (we went)fuisteis (y'all were) fuisteis (y'all went)fueron (they/you all were) fueron (they/you all went)